Turbidity arises in gases or liquids through the presence of dispersed materials. Turbidity can be ascertained by interaction between electromagnetic radiation and the measured medium, for example, either by measuring the weakening in intensity of a light signal which penetrates the medium (turbidimetry), or by measuring the intensity of light which is scattered on the dispersed particles (nephelometry). In the case of nephelometry, the intensity of the scattered light is ascertained at an angle (for example, 90°) to a measuring light beam radiated from an emitter.
The term, “light”, means here and in the following not only electromagnetic radiation of the visible spectral range, but also electromagnetic radiation of any wavelength, especially light in the infrared wavelength range.
In this context, the use of diodes as emitters and detectors is known. In such case, a light-emitting diode for producing a measuring light beam which lies within a suitable wavelength range (e.g. infrared radiation between 800 and 900 nm) is used as an emitter. As a detector, a photodiode can be correspondingly applied, which produces from the received, scattered light a detector signal (for example, a photocurrent or a photovoltage). The signal strength of the detector signal (here the photocurrent level or the level of the photovoltage), depends on the level of the intensity of the light reaching the detector diode, (thus, in the case of nephelometry, the intensity of the scattered light). This, in turn, correlates directly with the particle size and the concentration of the dispersed materials.
From the detector signal, a turbidity value can thus be determined for the measured medium to be examined. Because of the multiple scattering in turbid media, this turbidity value does not depend linearly on the concentration of the dispersed materials, (referred to in the following as “solids concentration”). Consequently, in order to be able to assign a solids concentration to a turbidity value, a calibration must be performed, for example, by means of comparison with a standard solution.
In the case of turbidity measurements in highly turbid measured media—for example, in slurries of a clarification plant—disturbances of the measuring—such as, for example, those caused by fouling of the turbidity sensor or by component fluctuations—can bring about corruption of the measurement results.
In the case of known methods for turbidity measurement, the use of two or more measuring paths serves to compensate for fouling of the measuring optics or for component fluctuations. Such method are described, for example, in DE 42 32.957 C2, DE 41 42 938 C2 and U.S. Pat. No. 5,140,168.
The turbidity measurement method described in these documents functions according to the four beam, alternating light principle. The sensor applied in such case possesses two emitters and detectors, which, in each case, lie opposite one another. When the emitters are operated one after the other, the two detectors in each case first receive two light signals (which are changed through interaction with the measured medium) from the first emitter and, thereafter, two corresponding signals from the second emitter. The detectors convert the received signals into detector signals, for example, into a photovoltage or a photocurrent. From the detector signals or from values derived therefrom, a turbidity value is ascertained.
In such case, a detector can receive a light signal which travels to the detector from an emitter along a direct propagation path, and the signal's intensity is weakened only by light losses from scattering. A detector can, however, also receive from an emitter along an indirect propagation path a light signal which has been scattered on dispersed particles of the measured medium.
A measuring of the turbidity of a measured medium according to the four beam, alternating light method thus includes a sequential excitation of the emitters, a producing of detector signals by the two detectors, and the ascertaining of a turbidity value from the detector signals.
For ascertaining the turbidity value, in the method described in the previously named documents, the signals, in each case, obtained along a direct propagation path upon excitation, in each case, of one of the emitters are multiplied with one another, and the resulting product divided by the product of the detector signals obtained along the indirect propagation path, in order to form a ratio. This ratio is a measure for the turbidity, and thus for the concentration of dispersed particles. In the concrete case of a suspension of solid particles in a liquid or gaseous medium, the concentration of dispersed particles is also called the “solids concentration”. Through this evaluation of the detector signals, component fluctuations or disturbances from fouling of the sensor can, up to a certain degree, be eliminated. Consequently, the four beam, alternating light method is, up to a certain degree, insensitive to such disturbances.
On the other hand, the low sensitivity of the turbidity value ascertained by means of the four beam, alternating light method to disturbances has the affect, that a self-diagnosis of the turbidity sensor on the basis this turbidity value leads only to inexact results, since only larger disturbances of the components or strong foulings make themselves noticeable in the turbidity value.
The sensor described in DE 41 42 938 C2 for turbidity measurements according to the four beam, alternating light method can, therefore, be checked between turbidity measurements with regard to the functioning of the light emitters and the light receivers. This reviewing cannot, however, occur during the measurement operation of the sensor, and, thus, also cannot be taken into consideration for an immediate plausibility checking of an ascertained turbidity value.